That's a common problem with backside because so many people teach you to cheat the spin and start turning your shoulders and looking back at the takeoff before you pop. I struggle with it like crazy. But a friend I ride with who absolutely slays it, never does that. Instead he counterwinds and stays looking straight forward until he is popping off the lip and then he spins more as a unit as versus letting the upper body go first and the legs following. Maybe go back to practicing 3s that way, with no cheating the spin or the head until you pop.
Take a look at Snowboard Addiction too Nev's videos can sometimes identify something you are doing wrong with your setup carve, windup and takeoff.